GHA: targeted cleanup — preserve BuildKit ccache across runs

- Replace nuclear 'docker system prune -af' and 'docker volume prune -af'
  with targeted removal of only the image we just built
- Add clean-gcc/clean-clang Makefile targets that remove specific image
  from both linuxkit cache and docker
- Add BuildKit cache pruning with configurable quota via BUILDX_KEEP_STORAGE_MB
  env var (defaults to 20000 MB) to prevent unbounded disk growth
- Print BuildKit disk usage before pruning for diagnostics
- Use buildctl inside linuxkit-builder container for cache management
- Switch pr-build.yml from pull_request_target to pull_request
- Remove unnecessary Docker Hub login from PR builds

Signed-off-by: Mikhail Malyshev <mike.malyshev@gmail.com>
(cherry picked from commit 04d9224)

Author: rucoder

Makefile.eve

@@ -84,7 +84,10 @@ docker-tag-clang: docker-tag-generate-clang
 push-gcc: push-image-gcc
 push-clang: push-image-clang
 
-.PHONY: kernel-gcc kernel-clang docker-tag-gcc docker-tag-clang push-gcc push-clang
+clean-gcc: clean-image-gcc
+clean-clang: clean-image-clang
+
+.PHONY: kernel-gcc kernel-clang docker-tag-gcc docker-tag-clang push-gcc push-clang clean-gcc clean-clang
 
 docker-tag-generate-%:
 	@echo "docker.io/$(IMAGE_REPOSITORY):$(BRANCH)-$(VERSION)$(DIRTY)-$*"
@@ -93,6 +96,10 @@ push-image-%:
 	$(if $(DIRTY), $(error "Not pushing since the repo is dirty"))
 	$(LK) cache push $(IMAGE_REPOSITORY):$(BRANCH)-$(VERSION)-$*
 
+clean-image-%:
+	$(LK) cache rm $(IMAGE_REPOSITORY):$(BRANCH)-$(VERSION)$(DIRTY)-$* 2>/dev/null || true
+	docker rmi $(IMAGE_REPOSITORY):$(BRANCH)-$(VERSION)$(DIRTY)-$* 2>/dev/null || true
+
 .PHONY: clean
 clean:
-	echo "Cleaning"
+	$(LK) cache clean